Underdrain filtration system with stamped perforations

ABSTRACT

An underdrain filtration system has improved perforation structure, the perforations being positioned to admit the filtrate into a plenum defined by elongated underdrain rib structures of the system. Instead of drilled or laser-cut holes or slits, the slits are formed by stamping discs from the stainless steel plate, such that the discs extend parallel to the surrounding flat area but are raised just sufficiently to define a plurality of slits under each disc, preferably two to four slits, each being an arc of slightly less than 180°, 120° or 90°, respectively. A pair of metal bridges remain to integrally connect the raised disc and the surrounding flat field. Consistent perforation width is achieved, at, for example, 0.010″ width varying by less than 0.001″ in either direction, improving consistency of pressure drop. In addition, the raised discs during backflushing of the underdrain create an outwardly diverging water flow pattern which is beneficial in clearing trapped particles from the filter medium in the vicinity of the underdrain perforations.

This application is a division of application Ser. No. 09/705,458, filedNov. 2, 2000, now U.S. Pat. No. 6,569,328.

BACKGROUND OF THE INVENTION

This invention concerns underdrain filtration systems for waterpurification or sewage treatment systems which employ a filtering mediumof stacked granular material retained above a tank floor by perforatedmetal underdrain structures. More particularly, the invention isconcerned with an improvement whereby the perforations in the metalunderdrain structures supporting the granular material are formed in anefficient manner so as to establish a high rate of consistency among theopening sizes, as well as promoting a better fluid flow pattern duringbackflushing.

An example of an underdrain filtration system is shown in applicationSer. No. 09/170,870, U.S. Pat. No. 6,090,284. As pointed out in thatapplication and patent, such granular filtration systems typically havecorrugated or undulating-shaped perforated underdrain structures, andthe systems are equipped to backwash the granular material covering theunderdrain units by water, and usually by air from an adjacent source.The assignee's U.S. Pat. No. 4,331,542 also describes an underdrainstructure for use with such gravity-fed granular filtration systems,fitted with water backflushing and air scouring.

As explained in those patents, the subject high rate gravity filtersusually have filter beds with gravel of varying sizes. Liquid from thetank or vessel flows downwardly by gravity through the filter medium andexits the filtration tank through openings in the underdrain structuresthat support the granular filtration medium. Typically the coarsestgravel of the medium is at the bottom, against the underdrain structure,with the gravel decreasing in size up to the finest gravel or sand,located at the top of the filter bed. Some filter beds are all sand, andthe bed may be covered with finely granulated anthracite coal or othertypes of filter material.

Backwashing and air scouring are important aspects of these types ofgravity filters, for cleaning the filters against clogging. Water or acombination of air (or gas) and water is passed up through the filterbed, in the opposite direction of normal filtration. Air may be from atube adjacent to the underdrain structures. Uniform distribution of thebackwashing fluids is a goal of most underdrain structures. Thisincludes the systems described in the above-referenced patents, as wellas some of the underdrain systems of the following patents: U.S. Pat.Nos. 4,659,462, 4,707,257, 5,015,383, 5,019,259, 5,156,738, 5,160,614,5,269,920, 5,332,497, 5,462,664, 5,489,388, 5,512,174, 5,639,384.

Application Ser. No. 09/170,870, U.S. Pat. No. 6,090,284, shows anembodiment of an underdrain structure wherein the water-passingperforations are in the form of narrow slits, small enough to support abed of sand without allowing the sand to pass through the openings. Slitopenings were made by laser cutting, as opposed to an earlier practiceof forming the openings as {fraction (3/16)}″ diameter holes, over whichgravel was placed.

The consistency of the opening sizes is important for proper waterdistribution in backflushing and balancing of pressure drop across theentire underdrain structure. Laser cutting is fairly expensive and hastypically not been capable of providing a highly consistent range ofopening width. The openings formed by laser cuts in the experience ofthe applicant ranged from about 0.008″ to 0.012″, a variation of 0.002″in both directions from the target size of 0.010″. The resultingdifferences in pressure drop tended to cause inefficiency in the actionof the backwashing water in cleaning the surrounding filter medium.Moreover, even if these slots could be formed perfectly at the targetwidth, the slots would not provide an optimum pattern of pressurizedbackwash water flow for filter cleaning, as does the system of theinvention described below.

SUMMARY OF THE INVENTION

The invention is an improvement in the fabrication and performance ofthe filter media-retaining underdrain. Instead of round holes orlaser-cut slots, the invention utilizes a machine punch process toproduce fine openings in a metal plate of specific thickness gauge,resulting in openings of a very consistent width. In one embodiment theopenings are 0.010″ in width and sufficiently small and consistent todirectly retain a filter medium (sand) ranging from 0.3 mm to 0.5 mm.The punched plate preferably is then folded into a trapezoidal shapesuch as shown in the above-referenced U.S. Pat. Nos. 4,331,542 and6,090,284. The trapezoidal underdrain structure defines a conduit forcarrying filtered water or wash water in a municipal water treatmentfilter or tertiary treatment in a municipal waste water treatmentfilter.

The stampings preferably are circular in shape, although they could beother curved shapes or polygonal, and produce a plurality of slots whichlie generally in a circuit defined by the space between a raised discand the surrounding flat area of the underdrain material. In onepreferred embodiment the raised discs are circular, with two to fourbridges between ends of adjacent slits, these metal bridges being leftremaining from the stamping of the discs. Two such bridges between twoarcuate slots are generally sufficient, but three bridges will providemore stability of the aperture sizes in the event of rough handling orother forces experienced during transportation, installation and use.

A very highly consistent slit width can be formed in this way, varyingless than 0.001″ in either direction, and thereby improving consistencyof pressure drop throughout the underdrain. As a result of the small andconsistent dimensions achievable with the process of the invention,underdrain structures can support the sand filter medium directly on theunderdrain plates, thus eliminating the space normally required forprogressively smaller gravel layers positioned over the underdrain tosupport the sand above, as in previous implementations.

A further benefit of the stamped-disc orifices is that the raised discsduring backflushing through the underdrain create an outwardly divergingwater flow pattern beneficial in clearing trapped particles from thefilter medium in the vicinity of the underdrain apertures.

Tests were performed on underdrain sections formed according to theinvention were tested for pressure loss. The pressure loss was found tobe much lower than head loss through underdrain structures withlaser-cut slots of the same total area of openings. This is due to theconsistency of the openings and the lack of irregularity along the edgesof the slots which are encountered with laser-cut slots.

It is thus among the objects of the invention to improve the structure,manufacturing procedure and cost of metal underdrains for granularfilter beds, through use of a stamping process that forms raised discsin the metal structure, defining slits arranged generally in a circuit.These and other objects, advantages and features of the invention willbe apparent from the following description of a preferred embodiment,considered along with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a granular filtration systemincluding an underdrain structure of the type to which this inventionrelates.

FIG. 2 is a view showing a flat metal plate punched or stamped to formraised discs and defining of slit openings in the plate, in accordancewith the invention.

FIG. 3 is a perspective view showing a raised disc and resulting slitopenings in greater detail.

FIG. 4 is a view similar to FIG. 2, showing a variation.

FIG. 5 is a perspective view showing an underdrain structure formed bybending a punched plate such as shown in FIG. 2.

FIGS. 6A to 6E are views showing other shapes which can be employed inthe punching of the discs to form the slit apertures.

FIG. 7 is a schematic elevation view showing a plate used over anexisting underdrain system to provide advantages of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the drawings, FIG. 1 shows in perspective major components of aliquid filtration system 10 partially in accordance with prior art andalso in accordance with features of the invention to be described below.The filtration system is formed as part of a tank typically of concreteand having a floor 12. This is a system of the type described in U.S.Pat. No. 4,331,542, as well as some of the other patents referencedabove. The filtration system, as described above and also in the '542patent, has a bed of granular material 14 which may be topped with alayer of bituminous material 16. Supporting the granular bed 14 is aperforated surface 18 at the bottom of the tank, preferably a corrugatedsurface as shown and as described in the '542 patent. Such a surfacepreferably is formed of a series of perforated longitudinal ribs 20which form triangular or trapezoidal cross-sectional shapes when placedagainst the floor 12.

In a preferred embodiment, as shown particularly in FIG. 5, eachperforated longitudinal rib 20 preferably has a horizontal flange 22 atleft and right extremities, these flanges of adjacent longitudinal ribsbeing slightly spaced apart in the completed underdrain unit 24, thusdefining troughs of the corrugations which are partly constituted by thetank floor 12 itself. As explained in U.S. Pat. No. 4,331,542, thelongitudinal ribs define inner spaces or chambers 26 which collectliquid filtrate passing through the granular bed 14 and through themultiplicity of perforations 28 in the ribs. This filtrate travels toand collects in a gutter or gullet 30 of the tank bottom.

As noted above, granular filtration systems and underdrain units of thisgeneral type are ordinarily equipped with liquid backwashing subsystems,and also pressurized air (or gas), sometimes called scouring air, whichis delivered adjacent to the underdrain perforations and to thebackwashing liquid to aid in the backwashing of debris from the filterbed. FIG. 5 shows a form of air scour subsystem wherein air isdistributed through a tubular structural member 30 which has airdistribution outlet holes or slits shown at 32. This tubular structuralmember 30 also serves to retain the underdrain structures 20 downagainst the tank floor 35, thus serving a dual purpose, as disclosed inthe above referenced application Ser. No. 09/170,870, U.S. Pat. No.6,090,284.

FIG. 2 shows in flat plan view a sheet of metal 38, preferably stainlesssteel in a thickness of about 1.6 mm or slightly under {fraction(1/16)}″, from which the perforations or apertures 28 are formed inaccordance with the invention. FIGS. 2 and 3 show that in oneembodiment, the apertures are formed by a stamping process, as by apunch press which stamps or punches out raised discs 40 from the flatsheet of material 38. The raised discs 40 formed in accordance with thisembodiment are generally circular, with diameters which may beapproximately ⅜″ to 1″. The discs are punched to a controlled depthwhich opens slits 42 between the disc and the surrounding flat field ofmetal, and these slits are highly consistent in width. A preferred widthfor this embodiment is about 0.010″.

The discs 40 are stably and rigidly retained on the flat metal sheet 38by metal bridges 44 which are left remaining in the stamping process.The stamping die is shaped so as to leave these metal bridges 44 and isprecisely formed to result in highly consistently-sized slits 42 servingas perforations for passage of filtrate. With the consistently formedopenings in sizes of about 0.010″ as is preferred for some applications,the granular material 14 can have sand piled directly on the underdrainstructure formed from the sheet metal plate 38, and an intermediate bedof gravel in gradation of sizes can be eliminated. This can save up toabout one foot of space in some types of filtration systems.

The metal stamping, a portion of which is shown in FIG. 2, is formedinto an underdrain structure which preferably is trapezoidally shaped asshown in FIG. 5. The bending of the stamped metal, along bend lines 46and 48, is done subsequent to the stamping of the apertures, in apreferred implementation of the method, since the stampings are moreeasily accomplished on a conventional press when the metal piece is incompletely flat form. However, a special stamping die could be providedto punch the discs after bending of the plate.

FIGS. 2 and 3 show stampings which form raised discs 40 connected by twoopposed metal bridges 44. However, as discussed above, these bridges canbe greater in number, such as three or four around a circuit of slits,so as to assure a more secure, rigid and stable connection between theraised discs and the surrounding flat metal area. Thus, in anotherpreferred embodiment three equally spaced bridges are left remainingfrom the stamping process, thus providing three arcuate slits which arespaced apart 120° and which are separated by the bridges. FIG. 4 showsan embodiment wherein raised discs 40 are retained to the surroundingmetal plate 38 with four such metal bridges 44. Thus, four arcuate slitsor punched slots 42 are formed as filtrate apertures, separated by thebridges 44. However, three such bridges are generally sufficient.

Although FIGS. 2, 3 and 4 show aperture slits 42 in a circular pattern,with circular raised discs 40 formed in the stamping process, thecircuit of slits and the shape of the disc need not be circular. It issufficient that the slits be arranged in some form of circuit, the slitsbeing separated by bridges, such that a raised disc of any practicalshape is formed in the punching or stamping operation. A circle is anefficient shape, but FIGS. 6A-6E shows other shapes, as examples, whichcan be employed. FIG. 6A shows schematically a series of six slits 42 ain a hexagonal pattern, with bridges 44 a between adjacent slits. Thus,a hexagonal raised disc 40 a is formed. FIG. 6B shows a square raiseddisc 40 b, with a pattern of four slits 42 b in a square arrangement,and this could be a non-square, rectangular arrangement.

FIGS. 6C, 6D and 6E show other shapes, including a triangular raiseddisc 40 c, with three slits 42 c in a triangular pattern; an octagonalarrangement in FIG. 6D, with an octagonal raised disc 40 d; and an ovalor elliptical shape in FIG. 6E, with a pair of slit apertures 42 edefined between the surrounding flat metal and a raised oval disc 40 e.Only two slits 42 e are shown at FIG. 6E, but there could be three orfour slits.

FIG. 7 shows schematically another application for the punched slotopenings formed in accordance with the invention. The drawing showsexisting underdrain structures 20 a as implemented prior to theinvention, and these typically have {fraction (3/16)}″ diameter holesfor the filtrate. As noted above, these require a support gravel systemabove the underdrain structure, for supporting the fine granule filtermedium such as sand. These existing underdrain systems can be fittedwith a flat plate 52 placed across the tops of and supported by the flattop portions 20 b of the existing underdrain plates or structures 20 a.This eliminates the need for a support gravel bed above the underdrainstructures, increasing the vertical height available for the actualfilter medium by about 12″, and thus improving the filtering capabilityof the existing filter.

The plate 52 has punched openings as described above (not shown in FIG.7). By leaving the existing underdrain structure 20 a in place, theillustrated arrangement makes this filter improvement very inexpensive.The flat plates 52 can be attached to side walls 54 of the filter systemvia an angled bend 55 in the plate, with stainless steel anchors 56 asshown, and attached to the tops of the existing underdrain structures 20a with stainless steel machine screws indicated at 58. Other methods ofsecuring can be used. Filter sand or anthracite can be placed directlyon top of the flat plates 52 with their very fine, preferably 0.010″punched openings. Because the plates 52 lie flat, they need to besomewhat thicker than the folded plates forming the underdrainstructures 20 described above, in order to support the filter medium andwithstand the forces generated by a high rate backwash without permanentdeforming.

The above described preferred embodiments are intended to illustrate theprinciples of the invention, but not to limit its scope. Otherembodiments and variations to this preferred embodiment will be apparentto those skilled in the art and may be made without departing from thespirit and scope of the invention as defined in the following claims.

I claim:
 1. A method for forming a filter structure and using the structure to support a bed of fine granular filtration medium, the method comprising: providing a flat sheet of metal, stamping the flat sheet of metal in a punch press which has a die which will punch out a series of raised discs from the surrounding flat area of the sheet of metal, such that at least two slits are formed as apertures between each raised disc and the surrounding flat area of metal, each disc being integrally connected to the surrounding flat area by metal bridges positioned between ends of adjacent slits and which are left remaining from the stamping of the discs, and the slits being of a controlled and predetermined width for supporting the fine granular filtration medium of preselected size, placing the flat sheet of metal stamped with the discs and slits in an underdrain filter system at a position supported above a floor to define a filtrate plenum between the sheet of metal and the floor, into which filtered liquid flows after passing through the slits, and placing the bed of fine granular filtration medium directly on the sheet of metal so that a liquid material can be passed down through the granular filtration medium to be filtered, with filtrate liquid flowing down through the slits in the plate.
 2. A method for forming a filter structure for use in supporting a bed of fine granular filtration medium, the method comprising: providing a flat sheet of metal, stamping the flat sheet of metal in a punch press which has a die which will punch out a series of raised discs from the surrounding flat area of the sheet of metal, such that at least two slits are formed as apertures between each raised disc and the surrounding flat area of metal, each disc being integrally connected to the surrounding flat area by metal bridges positioned between ends of adjacent slits and which are left remaining from the stamping of the discs, and the slits being of a controlled and predetermined width for supporting the fine granular filtration medium of preselected size, and bending the punched flat sheet of metal along a plurality of lines to form an elongated perforated metal rib structure to serve as an underdrain structure for an underdrain filter system.
 3. The method of claim 2, further including placing an array of such elongated perforated metal rib structures against a floor in an underdrain filtration system, with the rib structure extending upwardly from the floor to define within each rib structure a filtrate plenum between the rib structure and the floor, and placing directly on the array of elongated rib structures a bed of fine granular filtration medium such that the elongated rib structures support the filtration medium, such that filtered liquid passing through the granular filtration medium is filtered by the medium to form a filtrate which passes down through the slits in the elongated perforated rib structures. 